1. Field of the Invention
The present invention relates to a process for zeolite synthesis and to a device for implementing it. The present invention relates more particularly to the synthesis of zeolites of all kinds in a stirred medium with a view to improving the manner of formation of the zeolite crystals and their quality.
2. Background of the Art
To obtain zeolites, it is usual to form so-called crystallizing solutions or gels containing a source of trivalent aluminium, a source of tetravalent silicon, at least one alkali or alkaline-earth metal cation in hydroxide form, water and optionally an organic structuring agent. However, from these constituents it is possible to obtain quite different zeolites, such as offretite (OFF), mazzite (MAZ) or zeolite L (LTL) (S. Ernst and J. Weitkamp; Catalysis Today 19, 1994, 27-60) by varying their distribution or the subsequent hydrothermal treatment needed for the crystallization, or else the stirring of the medium.
Other syntheses are obtained from gels into which natural or synthetic crystalline aluminosilicates or even crystals of zeolites are introduced as replacement for amorphous sources of aluminium. Such substitutions have made it possible to obtain a homogeneous growth of the crystals of target zeolites, regardless of whether this is done in an unstirred medium or a stirred medium, because of the slow and uniform dissolution of these aluminosilicates or of these crystals of source zeolite in the synthesis medium. It has thus been possible to synthesize zeolites of mazzite type (Dwyer, U.S. Pat. No. 4,091,007, Fajula, U.S. Pat. No. 4,891,200, Cannan, U.S. Pat. No. 4,840,779 and Di Renzo, FR 2,651,221).
The prior art is rich in examples which show that small variations in the composition of the synthesis gel or in the crystallization conditions can result in the synthesis of zeolites which are very different in kind or even of amorphous compounds. Another factor which affects the growth of the crystals is the stirring (or the absence of stirring) of the synthesis medium, independently of the composition of the crystallizing solution or gel. Thus, in order to obtain a large number of zeolites, a person skilled in the art advises strongly against the use of stirrers during the crystallization of the zeolite, both during the maturing stage and during the crystal nucleation stage. It has been ascertained that, depending on the composition of the crystallizing solution or gel, it is possible to see appearing either a mixture of faujasite and of gmelinite in a static medium, or of zeolite P in a medium stirred with an anchor stirrer (D. E. W. Vaughan, Chem. Engin. Progress, 84(2), 1988, pp 25-31). In fact, the high shear rate generated by the stirrer localized only in a portion of the synthesis medium considerably modifies the process of germination and of growth of the crystals of zeolites and their nature, even if the initial composition of the gel is the same. In parallel, in the case of zeolites which are usually prepared in a stirred medium, like, for example, faujasite, a lowering has been noted in the crystallinity of the faujasite obtained when the stirrer speed and hence the local shear are increased (R. M. Barrer, Hydrothermal Chemistry of Zeolites, Academic Press, 1982, p. 171).
It is for the abovementioned reasons that many syntheses of zeolites are carried out statically, that is to say without stirring, because this promotes the slow growth of the crystals and the appearance and stabilization of zeolite phases which are thermodynamically not very stable. Moreover, it has not yet been possible to reproduce synthetically many natural zeolites in the pure state or as by-products of synthesis of other zeolites, regardless of whether this is in a stirred medium or a static medium.
Furthermore, it is well known to the manufacturers of zeolites that the transposition of some syntheses to an industrial scale is difficult or even impossible. It often results in low zeolite yields and in degrees of crystallinity which are much lower than those generally obtained on the laboratory scale (D. W. Breck, Zeolite Molecular Sieves, John Wiley and Sons, 1974, p. 725-731).
For the above reasons it is impossible to synthesize zeolites by processes that combine two types of stirring including anchor stirring and stirring by means of a propeller placed in a cylinder in perpetual motion, as proposed in patent U.S. Pat. No. 1,115,791, since these stirring actions induce strong variations in shear at the anchor and the edges of the cylinder in motion, which do not promote the maturing and the stabilization of the crystal phases.
The present invention is therefore aimed at a process for zeolite synthesis which makes it possible to stabilize more easily some zeolite phases that are highly unstable, even in static operation, to improve the quantitative yields of synthesis in relation to those obtained in a static or conventional stirred medium while shortening the synthesis time by virtue of better control of the transfers of matter and of heat in the synthesis gel or solution, and thus greatly to increase the degree of crystallinity of the zeolites. In addition, the process aimed at can be easily transposed to an industrial scale.